1) Field of the Invention
The present invention relates to a Raman amplifier suitable for use in an optical transmission system.
2) Description of the Related Art
In a long-distance optical transmission system, transmission is heretofore performed using an optical regenerative repeater which converts an optical signal into an electric signal, then performs retiming, reshaping and regenerating of it. An optical amplifier, which amplifies an optical signal without converting it into an electric signal and repeats the optical signal, currently comes into practice, and an optical amplification repeater transmission system using this optical amplifier as a linear repeater is under examination at present.
Namely, by replacing an optical regenerative repeater used in such an optical transmission system with an optical amplifier repeater as a linear repeater as above, it is expected that the number of parts in the repeater is largely decreased, the reliability is secured, and the cost is largely decreased.
With a recent increase in quantity of information transmitted over a network, there is a demand for a large capacity of an optical transmission system. As a method realizing a large capacity of the optical transmission system, the wavelength division multiplex (WDM: Wavelength Division Multiplex) optical transmission system, which multiplexes optical signals having not less than two different wavelengths and transmits them, attracts attention.
The WDM optical amplifier repeater transmission system, which is a combination of the WDM optical transmission system and the optical amplifier repeater transmission system, can collectively amplify optical signals having not less than two wavelengths using an optical amplifier without converting them into electric signals, which can realize a large-capacity, long-distance transmission in a simple, low-cost structure.
As a repeater of the above optical amplifier repeater transmission system, there is generally used, for example, an erbium-doped optical fiber amplifier (EDFA). A 1.55 μm band (C-band), for example, is used as a gain wavelength band of the EDFA. On the other hand, a 1.58 μm band (L-band) is used as a gain wavelength band of GS-EDFA (Gain shifted-EDFA) whose gain band is shifted to a longer wavelength. Both of EDFA and GS-EDFA have wavelength bands of not less 30 nm. It is therefore possible to realize a band of not less than 60 nm by using both the two signal optical wavelength bands by a multiplexer/demultiplexer for C-band and L-band.
In order to realize a large-capacity long-distance transmission system, increase of the signal optical wavelength band is essential. In order to realize a wide band, it is vitally examined to apply a Raman amplifier using Raman scattering as a repeater, in recent years.
Raman amplification gives pumping light to an optical fiber to provide a gain on a longer wavelength's side than a wavelength of the pumping light. Depending on a composition of an optical fiber giving pumping light, a frequency smaller by about 13.2 THz than a pumping light frequency can be a gain peak optical frequency, for example. When converted into a wavelength, the Raman gain peak wavelength is a 1.55 μm band shifted by about 100 nm from the pumping light wavelength in a 1.45 μm band.
In order to realize an amplifying function at a signal light wavelength required in Raman amplification, it is essential to set a pumping light wavelength in consideration of such a Raman shift frequency. It is alternatively possible to flatten a gain wavelength characteristic of Raman amplification using plural kinds of pumping light having different oscillation center wavelengths.
In a Raman amplifier, a pumping light power and its oscillation wavelength are adjusted to secure about 100 nm as its gain wavelength band width as disclosed in “Y. Emori, et al., ‘100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes’, OFC' 99, PD19, 1999.”
FIGS. 19 and 20 are block diagrams showing known Raman amplifiers. A Raman amplifier 100A shown in FIG. 19 comprises a pumping source 101, a multiplexer 102A and an optical fiber 103. A Raman amplifier 100B showing in FIG. 20 comprises an optical circulator 102B along with a pumping source 101 and an optical fiber 103 similar to those of the Raman amplifier 100A shown in FIG. 19.
In each of the Raman amplifiers 100A and 100B shown in FIGS. 19 and 20, the pumping source 101 generates pumping light P1-PK having different wavelengths. The multiplexer 102A or the optical circulator 102B propagates the above pumping light P1-PK to the optical fiber 103 in the opposite direction to signal light S1-SL. The signal light S1-SL is counterpropagating-pumped by the pumping light P1-PK generated by the pumping source 101, Raman-amplified, and outputted through the multiplexer 102A or the optical circulator 102B.
The signal light S1-SL and the pumping light P1-PK in each of the Raman amplifiers 100A and 100B have a relationship in arrangement of wavelengths as shown in FIG. 21. Namely, as shown in FIG. 21, wavelengths are arranged such that a signal light wavelength band does not mix with a pumping light wavelength band.
In order to obtain a larger capacity of the transmission capacity in the Raman amplifier, it is necessary to widen the signal light wavelength band width. For this purpose, the signal light and the pumping light may be mixed within a certain band as shown in FIG. 22, for example. In the arrangement of wavelengths shown in FIG. 22, sepctra PQ+1-PM that is a part of M wavelength spectra (hereinafter simply referred as spectra, occasionally) of the pumping light P1-PM are mixedly present within a band of spectra S1-SR that are a part of spectra S1-SN of the signal light.
In the case of the arrangement in which the signal light and the pumping light are mixedly present inside a certain band, there occurs a problem that degradation of the transmission characteristic generates due to linear crosstalk or non-linear crosstalk as described below.
As denoted by PM-1 in FIG. 23, a wavelength spectrum of the pumping light P1-PM has a width 101a extending on the longer wavelength's side and the shorter wavelength's side with a center wavelength PM-1 being the center. For this, the wavelength spectrum of the pumping light is widely overlapped on wavelengths arranged as the signal light optical wavelengths inside a band 101b in which the signal light wavelengths and the pumping light wavelengths are mixedly arranged.
In this case, when Raman amplification is performed by counterpropagating pumping like the Raman amplifier 100B shown in FIG. 20, Rayleigh scattering light at a wavelength of the pumping light overlapped on the signal light propagates in the same direction as the signal light, thus becomes noise light to the signal light, which causes degradation of an optical SN ratio of the signal light due to linear crosstalk, as shown in FIG. 24. In the case of copropagating pumping, the pumping light travels in the same direction as the signal light when a band of the pumping light wavelength overlaps on a part of a band of the signal light, so that wavelengths of the signal light overlap on the pumping light wavelength. This degrades an optical SN ratio of the signal light due to linear crosstalk, as well.
The optical SN ratio of the signal light whose wavelength spectrum is overlapped by the pumping light significantly degrades due to the following non-linear crosstalk.
Namely, since a power of the pumping light is extremely large, four-wave mixing occurs between the signal light and the pumping light, and in the pumping light. In concrete, since the pumping light of Raman amplification is extremely larger than the signal light, four-wave mixing occurs between the signal light and the pumping light, which causes a transmission characteristic due to the following non-linear crosstalk.
Four-wave mixing is light newly generating due to mixing of optical frequencies, which satisfies the following conditional equation (1) or (2) (refer to FIG. 25):f4=f2+f3−f1  (1)f4=2f3−f2  (2)wherein, f4 is a frequency of four-wave mixing light newly generating, and f1, f2 and f3 are frequencies of existing light. When f2 is arranged at the signal light and a wavelength f3 has a pumping light component, for example, new light at a wavelength f4 obtained by the equation (2) generates. When the signal light is arranged at the wavelength f4, degradation of the transmission characteristic of the signal light occurs.
As cited above an existing optical power threshold value when the four-wave mixing light generates is extremely low. Namely, it can be considered that the above four-wave mixing generates due to an optical power of a pumping light component away from the center wavelength. When a beat noise caused by the four-wave mixing light newly generating due to such the four-wave mixing and the existing signal light enter into a baseband of the optical receiver, degradation of the transmission characteristic much larger than the power crosstalk occurs, which becomes a factor of limitation exerting a large effect on the WDM transmission.